Amp stereo mode suddenly not working but bridged mode is working

[Peter Truverez]

When I work on an amp that is driving me nuts, I sometimes turn the amp 180 degrees. Like work on it from th rear instead of from the front. This gives me a new viewpoint. SOunds silly, but when something basic looks to be wrong and I cannot find it, many times it is because I looked at something long enough that it looked "right" even though it wasn't.


When you said this earlier, it concerned me, since you are automatically losing one side of a stereo signal when in bridged mode. You can mono up the source and feed it to one input, but otherwise that second input does nothing. SO I am not sure how we would be losing anaything.

I was speaking from my car audio experience where majority of bridgeable stereo amps maintain stereo inputs when in bridged mode. I know you can sum the stereo signal to a mono input, i've built cables to do this very thing.

 

[Enzo]

I don't work in car audio, but are not most car amps bridge mode to start with? In other words a simple stereo is actually two pairs of bridge mode amps? DO car amps have a stereo/bridge switch?

 

[Peter Truverez]

I don't work in car audio, but are not most car amps bridge mode to start with? In other words a simple stereo is actually two pairs of bridge mode amps? DO car amps have a stereo/bridge switch?

I've never owned a bridgeable car amp that had a mode select. You simply wire it for bridged or wire it for stereo. They've had mono or stereo input selection but no selection for output.

 

[weltersys]

I understand the amp is not a mixer but it's not difficult to sum a stereo signal to mono. I've done it very easily while building a 3.5mm stereo jack to XLR adapter.

Your input wiring (common to all four amps) is likely the problem.

You can "easily" connect (short circuit) the tip and ring of a stereo 3.5mm (or 1/4") TRS together, then wire the shorted pair to the XLR pin 2 (+), and the sleeve to XLR pin 3 (-) to sum to mono.
Not a good choice, the tip and ring should be isolated with resistors.

To function properly, a 3.5mm stereo jack to XLR adapter would require two XLR connectors, one for left, one for right.

The standard connection of a 3-pin XLR to a 1/4" TRS (AKA stereo jack plug) is:
pin 1(shield) to sleeve, pin 2(+) to tip, pin 3(-) to ring.

If a mixer stereo headphone out was used to drive the amp input using that configuration, the L/R of a mono signal (exactly 180 degrees out of phase) would cancel if driving the amp XLR switched to "stereo", while the inverting "bridge mono" connection would not.

https://qscprod.force.com/selfhelpportal/s/article/How-to-Connecting-stereo-outputs-to-mono-input